If you want it to look naff then repairing it is the way to go.
First things first though, you wanna cover that hole with anything for starters, once water gets into that foam it will eventually turn it to mush and it will rot.
Plus eveytime you sit on it the chances are you will get wet.
Or back in the real world, get it recovered properly.
As suggested, get a new cover off Ebay, you will need a very heavy duty stapler, and preferably someone to help you to do it as it can be fiddly.
My old neighbour did mine, and she used to be an upholsterer for Rolls Royce, but i still helped her when she replaced it for me.
If you're not confident doing it, get it done properly in a shop, because if you make a right pigs ear out of it, it can really spoil the bikes appearence, as can strips of Gaffer tape covering a hole.
You could try and find another seat going cheap, but chances are it'll have the original seat cover on as well and will also need doing again in the near future.
http://www.ebay.co.uk/itm/Yamaha-FZS600-Fazer-Seat-Cover-1998-to-2003-/302012044502?hash=item46515214d6:g:lWUAAOSwl8NVbi3W
